The bleaching is worst in the most pristine and remote parts of the reef north of Cairns, according to Terry Hughes, convenor of the National Coral Taskforce. “It’s the jewel in the crown of the Great Barrier Reef and it’s now getting a quite a serious impact from this bleaching event,” he said. “The northern reefs are bleaching quite badly now.”

Hughes said it appeared there was some coral death occurring in northern reefs.

Russell Reichelt, the chairman of the Great Barrier Reef Marine Park Authority said the area around Lizard Island, 250km north of Cairns, and sites further north, had fared the worst.

The world is currently in the grips of the third global coral bleaching event. Coral bleaches when water temperatures are raised above a certain threshold for an extended period of time.

Climate change will lead to deformed and virus-hit coral reefs

Read more

Hughes, director of the ARC centre of excellence for coral reef studies at James Cook University, said although the strong El Niño occurring now is partly to blame for the bleaching event, the real culprit is global warming caused by carbon emissions.

“These massive thousand-kilometer bleaching events didn’t happen thirty years ago,” he said. “No-one ever recorded a mass bleaching event in the 40s, 50s, 60s, 70s, until the middle of the 1980s … and the Great Barrier Reef didn’t bleach until 1998 for the first time.”

“The baseline temperature on the barrier reef has gone up between a half a degree and a full degree depending where you are on the great barrier reef. Bleaching happens once coral sits in water a degree or two above the normal summer maximum for a month or so.”

We're heading to a point where the Great Barrier Reef might bleach during every El ​​Niño, risking its very existence.

Hughes says we are heading towards a future where the Great Barrier Reef might bleach during every El Niño , which will put its existence at risk.

Based on the severity of bleaching reports, the Great Barrier Reef Marine Park Authority has lifted its bleaching warning from Response Level 1, which means mild and widespread bleaching, to Response Level 2 (severe and local).

The silver lining in the announcement is cloud cover and cooler temperatures have created safer conditions for two thirds of the reef – most areas south of Cairns.

“In the last couple of weeks we’ve had a lot of cloud cover in the middle and the south, so the danger period has basically passed for the reef south of Cairns.”

But the announcement has led to calls for immediate action from conservation groups around Australia.

Greenpeace called for Queensland to limit its coal exporting. “In the two weeks since the level 1 response plan began, the Queensland government has allowed some 8m tonnes of coal to be exported straight through this delicate ecosystem. This coal will be burnt overseas, driving climate change, warming our oceans and contributing to coral bleaching,” said Shani Tager, Greenpeace Reef Campaigner.

Imogen Zethoven, the Great Barrier Reef campaign director at the Australian Marine Conservation Society said: “As the bleaching on the Reef continues to intensify we need an urgent response from the Turnbull government to avoid widespread bleaching happening repeatedly in the future.”

“This bleaching event has revealed the true cost of approving more coal mines, more coal export port terminals and refusing to listen to the warnings ... The solutions are clear, we must make a rapid transition from mining and burning coal to 100% renewable energy.”

WWF-Australia called for an injection of $1m in federal funding for coral monitoring.

“GBRMPA is clearly concerned and is being proactive in lifting its response level and we support them in taking this action,” said WWF spokesperson Richard Leck.

“Surveying the impact of this bleaching event requires significant additional funds because of the challenges of getting scientists into the field over enormous distances.”